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Early Season Drought and Heat Stress on Small Grains (06/11/20)

Rainfall during the past month has been below normal. The abundant rainfall last fall that filled the soil profile has helped to mitigate the effects of these deficits in most of the state.

Rainfall during the past month has been below normal. The abundant rainfall last fall that filled the soil profile has helped to mitigate the effects of these deficits in most of the state. Nevertheless, about a third of the state (primarily in the west) is classified as being under moderate drought conditions. Not surprisingly, last week I received a call from a concerned grower wanting know the impact of drought stress on small grains during early vegetative development. His crops were particularly hard hit since there were being grown on sandy ground.

Most small grain crops were planted later than normal this season and are still relatively small. Small plants with limited leaf area are not heavy water users. The early planted small grain crops, however, have nearly canopied and during some of the warmer days last week were using nearly a quarter inch of water a day. This rate of transpiration can rapidly deplete soil moisture in the absence of timely rain. Limited water availability to young plants is exacerbated by the fact that their roots systems are still developing and are limited in the amount of soil moisture they can exploit. Therefore, it is not surprising to learn that some field have exhibited symptoms of drought stress. If a plant shows a loss of turgor (wilting) during the early vegetative stages of development its ability to photosynthesize is reduced and yield potential can be impacted. Stress during the pre-jointing stages reduces the number of tillers that are produced and the size of the spikes that do develop. Established tillers may be lost if drought stress intensifies as the plant develops further. Dry soil around the crown reduces the plants ability to develop adventitious roots which are the primary roots for accessing water and nutrients as the plant develops beyond the seedling stage. The literature I reviewed while preparing a response to the concerned grower suggested that severe drought stress (visible wilting) during vegetative stages of development (assuming that it will not persist beyond a week or two) could reduce yields by 25%. The actual losses will depend on the severity and duration of the stress.

As concerning as the drought stress discussed above is the impact of the recent unusually warm day and night temperatures. The ultimate size of the spike can be significantly reduced by warm temperatures (see Figure 1). Small grains, however, are able to compensate to some extent for these reductions if conditions are favorable during the rest of the season. They can add extra kernels and add weight to kernels that do develop in the spike. Early heat and drought stress may have taken the top off the yield potential of the crop in some areas of the state, but with favorable conditions going forward there is still potential for the development of a reasonably good crop; stress during vegetative development is typically less impactful than stress during grain filling.

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Joel Ransom

Extension Agronomist, Cereal Crops

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